Developing devices heretofore proposed for the above application include one in which toner supplied from a toner cartridge is caused to flow down by a toner supply roller and, then, mixed with a developer by an agitating roller. In this type of developing device, if developer collected from a developing roller is fed toward the agitating roller without any restriction, the developer is caused to sequentially accumulate in a wedge-shaped space which is defined between the agitating roller and the wall of a casing of the device. Then, the accumulated developer blocks toner being transported by the toner supply roller, resulting in the toner being apt to be collected to form a mass without being dispersed in the developer. Should such a mass of toner be bodily mixed with the developer at one time due to vibrations, shocks ascribable to the start and stop of a drive source for driving the developing device, or other causes, it would smear the background of a copy or be scattered out of the developing device to contaminate the interior of the image-forming apparatus.
The agitating roller is usually made up of a spiral blade which extends spirally along the axis of the roller and a plurality of radial blades, each extending radially from the axis of the roller. The spiral and radial blades cooperate to define some chambers for transporting the developer. When the roller having such a configuration is rotated to agitate the developer in both the axial and circumferential directions, it often occurs that the developer entering some of the chambers forms a film on the outer peripheral surface of the roller as time elapses. As a result, the developer becomes packed fast in the chambers. Especially, such a phenomenon is quite liable to occur when the viscosity of the developer is high. Packing of the developer in the chambers as mentioned deteriorates the agitating ability of the agitating roller and invites low density, irregular density, and other problems in reproduction.